Balcony Junction Psi Value Calculator (E8 / E24)

E8 / E24 SAP / Approved Document L reference junction

Schematic cross-section of the balcony junction (E8 / E24) showing the structure, insulation line and the heat-bridge path — Schematic of the balcony junction (E8 / E24) — not to scale. Model your actual build-up in ΨMonkey.

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What the balcony junction is

The balcony junction is where a balcony or canopy structure connects to the building — often a concrete slab or steel members cantilevering out through the external wall.

Why it matters

A cantilevered slab or steel that passes straight through the insulated envelope is one of the strongest thermal bridges in any building, carrying heat directly outside along its full length unless a structural thermal break is used.

How to model it

Model the floor slab or steel, the wall and insulation it penetrates, and any structural thermal break, then solve for the Ψ-value and the f_Rsi at the internal corner.

Without a thermal break the Ψ-value is high; a proprietary structural break dramatically reduces it — and the calculation shows the difference.

Related build-ups: the U-values either side of this junction

The Ψ-value at the balcony junction depends on the walls, floors and roof around it — so the first step is knowing the U-values of those elements. Typical figures for the build-ups that pair with this junction:

Model your exact build-up and read its U-value with the free U-Monkey calculator (PDFs included free), then come back here and solve the junction in ΨMonkey.

Free to model — no subscription

ΨMonkey isn't a monthly licence or a per-seat subscription. Build the balcony junction on the grid, solve it, and read the Ψ-value and f_Rsi on screen for nothing. You only pay £25 +VAT (£30 inc VAT), once, when you need the Building Control-ready PDF for that junction — so calculating a single detail never means signing up to anything. Calculated psi values are often the cheapest route to a SAP 10 pass, so the maths can pay for itself.

Frequently asked questions

What is the balcony junction (E8 / E24)?

The balcony junction is where a balcony or canopy structure connects to the building — often a concrete slab or steel members cantilevering out through the external wall. A cantilevered slab or steel that passes straight through the insulated envelope is one of the strongest thermal bridges in any building, carrying heat directly outside along its full length unless a structural thermal break is used.

How do I calculate the balcony junction psi value?

Model the floor slab or steel, the wall and insulation it penetrates, and any structural thermal break, then solve for the Ψ-value and the f_Rsi at the internal corner. ΨMonkey solves it to BS EN ISO 10211 and returns the Ψ-value (W/m·K) and the f_Rsi temperature factor on screen, free.

Is it free, and is there a subscription?

No subscription and no licence — modelling the junction and reading off the Ψ-value is free in your browser. You only pay £25 +VAT (£30 inc VAT) when you export the Building Control-ready PDF for that junction.

Can I use the result in a SAP or HEM assessment?

Yes — a calculated Ψ-value to ISO 10211 can replace the SAP default for this junction, and Without a thermal break the Ψ-value is high; a proprietary structural break dramatically reduces it — and the calculation shows the difference.

Other junctions

ΨMonkey models any junction. Jump to another common one:

Balcony Junction Psi Value Calculator